1. Field of the Invention
The present invention relates to thermal printers, and more particularly, to thermal printers that employ a capstan drive system to advance receiver paper through the printing station
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 4,710,783 describes a thermal printer apparatus that uses a dye transfer process to form an image on a receiver paper using a multi-colored thermal transfer ribbon from which dye is transferred by heat generated by a thermal print head. The thermal print head is formed of, for example, a plurality of individual thermal heat producing elements, often referred to as heating elements. The receiver paper and the thermal transfer ribbon dye carrier are generally moved relative to the print head and a platen roller at the printing station. The receiver paper is repeatedly fed through the printing station between the print head and platen by the forward and reverse rotation of the platen and/or capstan and roller drive assemblies while the ribbon is advanced to present the three dye transfer colors, thus performing multi-color printing by the successive registration of the three color images as a single print image on the receiver paper.
As described more completely in the above-referenced '783 patent, incorporated herein by reference in its entirety, the print head is organized into a plurality of groups of heating elements that are capable of being energized for predetermined time periods that determine the gray scale of an image pixel transferred. Thermal dye transfer printer apparatus offer the advantage of true "continuous tone" dye density transfer. By varying the heat applied to each heating element to the carrier, a variable dye density image pixel is formed in the receiver. When a particular heating element is energized, it is heated and causes dye to transfer (e.g., by sublimation) from the carrier to the image pixel in the receiver paper image frame. The density, or darkness, of the printed dye is a function of the temperature of the heating element and the time the carrier is heated by that element. In other words, the heat delivered from the heating element to the carrier causes dye to transfer to an image pixel of a receiver. The amount of dye is directly related to the amount of heat transferred to the carrier.
As mentioned above, thermal printers successively overlay color dyes to form a full color image onto the receiver paper. Alignment of each successive color is crucial for good image quality. Capstan drive systems rely on a paper nip to drive the receiver paper past the print head and platen for each successive color. The capstan is intended to precisely drive the paper past the head in a synchronized manner with aligned printing of the linear array of the heating elements of the print head which are themselves individually actuated by digital image line data in storage buffers that are successively loaded for each line with digital data from memory registers of the microprocessor-based control system depicted, for example, in the above-incorporated '783 patent.
The misregistration of the individual lines of the successively transferred color images is more or less noticeable depending on the content of the image being printed. Slight misregistrations of the successive image pixels of a pictorial scene are usually acceptable. However, even slight misregistration of the successive cyan, magenta and yellow image pixels forming black printed text may cause a halo effect of the misregistered colors at the borders of the black characters. Often it is desirable to print both pictorial scenes and alphanumeric characters as part of the same printed image, and misregistration may be only apparent in the printing of the characters.
Misregistration occurs from errors between the motion of the paper and the line placement of the head for each successively transferred color image. Since the receiver paper is a non-rigid structure (i.e., like a rope), the paper must be maintained under near constant tension to ensure accurate motion. Paper under constant tension will maintain a predictable path through the head platen nip and this ensuring good synchronization between the paper motion and the line placement.
Various types of driving systems have been proposed to prevent misalignment of the color planes in either the front to back or side to side direction relative to the thermal print head. In the case of the standard capstan and pinch roller drive systems presently employed in certain thermal printing apparatus, shifts in the color planes occur due to uncontrolled back tension on the receiver paper. Capstan and pinch roller systems rely on the friction of the capstan to drive the receiver sheet past the thermal print head to form an image made up from a matrix of pixels, and if the slip occurr on the friction interface between the capstan and receiver interface, color misregistration will occur. Color misregistration can also occur if a speed differential exists between the capstan roller and pinch roller, because the speed differential creates a shear force on the receiver paper that will result in slip. Capstan drive systems always intend the capstan roller to drive the receiver sheet and the pinch roller to be the follower; thus the pinch roller should not induce any shearing force to the receiver paper that could cause receiver slip. The present capstan drive systems do not prevent the pinch roller from inducing shear forces to the receiver paper. In addition the current systems do not allow shear forces to be relieved on the capstan roller. Thus, the induced shear forces on the paper from the capstan roller will cause slippage of the receiver paper in the nip area, and this slippage will show up as color misregistration.
Moreover, the uncontrolled back tension condition occurs because of the variability of the coefficient of friction at the head/receiver interface during printing. A stick/slip phenomena occurs at the head, due to the various levels of heat employed to create different density levels of the individual pixels. Therefore, the total tension on the receiver paper at any given point includes the slippage of the receiver paper in the nip area and the variable force of the head friction, and, as a result, the total paper tension varies during and from color pass to color pass, thus resulting in color misregistration.
Constant tension control can be sought through an additional mechanism placed upstream from the print head as set forth in U.S. Pat. No. 4,642,659. The printer drive apparatus disclosed in the '659 patent employs a hard capstan roller and a softer pinch roller to form a driving nip to transport the receiver paper through the head and platen interface at the print station. The image forming method comprises multiple passes through the print station to transfer each color dye image to the receiver. For example, a yellow, magenta, cyan and/or black dye pass for each printed image is made. In the '659 patent, the tension mechanism creates a back tension on the receiver greater than the force disturbance created during the printing process by virtue of the additional upstream capstan roller and pinch roller.
Other mechanisms for providing constant tension in the normal and reverse direction of the receiver through a print station employing combinations of hard and soft rollers and/or platens in both thermal transfer printing and in other printing technologies are disclosed in U.S. Pat. Nos. 4,502,804, 4,720,714, 4,834,277, and 4,957,378. Typically, in these patents, the platen roller at the print station is driven along with upstream and/or downstream capstan rollers which may operate as a tension roller to apply tension to either discrete sheets of receiver paper moved through the printing station or the continuous web of receiver paper moved bidirectionally therethrough from a paper supply reel.